Patent Document 1 discloses an A/D convertor array for image sensor. The A/D converter shown in FIG. 11 of the above document is configured to perform noise cancellation of reset noise with use of capacitors (C1, C2, C3) and an inverting amplifier and to subsequently perform cyclic A/D conversion with use of the capacitors (C1, C2) and the inverting amplifier, and the capacitor (C3) is not used in the cyclic A/D conversion.
Patent Document 2 discloses an A/D converter with a noise cancellation function. In the A/D converter shown in FIG. 4 of this document, a differential input/differential output amplifier is connected via a plurality of switches to capacitors (C1, C2, C3, C4 and C5). The capacitor (C5) determines the gain of the amplifier. This A/D converter necessitates five steps for noise cancellation as shown in FIG. 6 of the present document. A reset level is stored in the capacitor (C1), and a signal level is stored in the capacitor (C2). One terminal of the capacitor (C1) and one terminal of the capacitor (C2) are connected to differential inputs, respectively, and the other terminals of the capacitors C1 and C2 are connected to each other, thereby generating a difference signal at an output of the amplifier, and this signal indicates the difference between the reset level and the signal level. Next, connections of the capacitors (C1-C4) to the amplifier are switched through a plurality of switches, for cyclic A/D conversion, and the cyclic A/D conversion of this difference signal is carried out in two steps. The cyclic A/D conversion is repeatedly carried out to obtain an A/D conversion value with reduced random noise.    Patent Document 1: Japanese Patent Application Laid-open No. 2005-136540    Patent Document 2: Japanese Patent Application Laid-open No. 2007-104655